Kerbal Space Program: Part 3 – Advanced Aerodynamics and Hotkeys

We’ve been requested to go over what the Center of Mass is and how its used in KSP as well as an explanation on how to use hotkeys, so here we go!

So what is “Center of Mass”?

The center of mass, abbreviated CoM, or center of gravity, is the location of an object where all mass is equally distributed around it. It is important to balance a craft to prevent it from getting out of control. Planes should have a center of mass slightly in front of the center of lift. For all craft the thrustvector should point to, point away from or go through the center of mass.

It is possible to show the center of mass in the Vehicle Assembly Building and Space Plane Hangar, but not while in flight. Also it is not possible to view the center of masses of the different stages. This is the only information about a center which is only a point and not also a vector.

Unlike in real life, the center of mass of two celestial bodies called the barycenter, e.g. Kerbin and Mun, is always in the center of the orbited body, which is in the example Kerbin.

It’s also important to know about the Centre of Thrust as it works in tandem with the Centre of Mass.

The center of thrust or thrustvector is the direction in which the thrust is acting and on which point the thrust acts on the craft. The thrustvector and center of mass should be in one line to minimize steering to a minimum.

Placing engines above the center of mass doesn’t make a craft more stable, which is usually called pendulum rocket fallacy. In Kerbal Space Program such craft might be more unstable, as thrust vectoring needs to be applied inverted, when the engine is acting above the center of mass. So one way is to deactivate thrust vectoring, which makes rocket hard to steer especially in atmosphere. Another way would be to control in the other direction. To prevent the systems to counteract, torque ofreaction wheels needs to be deactivated and all control surfaces needs to be removed.

So now that you’ve had the technical description of what the Centers of Mass and Thrust both are, lets break it down to why it matters to you!

In general you’ll want the CoM to be around the middle of your spacecraft. The reason for this is to balance weight distribution so that you’re not overly top or bottom heavy in order to avoid tilting during takeoff or loss of control (spinning) while in space. Now the CoM doesn’t have to be exactly in the middle of the spacecraft, just fairly close.

If you’re building a craft with multiple engines on it, you’ll need to position them in such as way that your Center of Thrust goes or very closely in line with your Centre of Mass. If you find on launch that your craft stars banking in one direction without you telling it to do so, it’s probably because you’re off center between your CoM and CoT.

If, due to the nature of the design of your craft, you’re unable to get the CoT and CoM inline, all is not lost. Try adding some control surfaces like tail-fins to various parts of your fuselage (the part you plan to let fall back to Kerbin). This might be enough to help you get out of the atmosphere and in to orbit, but i can’t promise you a smooth ride! Once you’re in space, careful thrusting can get your mission accomplished. Try avoid sudden large bursts of thrust as you’ll tend to spin out of control in space if your CoT and CoM are off.

All of this should be enough to take care of your CoT and CoM questions for spacecraft. When we look at planes and space planes we’ll have to reexamine these again with the Center of Lift thrown in too!

And now for the Hotkeys (action groups)!

There are some predefined action groups, such as the G key for landing gear, the B key for brakes etc.

You can also setup custom action keys with the 1-9 keys on your keyboards! For example, if you wanted to add some solar panels and set it up so that you can deploy and retract them all at the touch of a button, you can do it with the action groups!

To try this out, stick on some solar panels with the 4x item attacher to any ship you’ve built. Then, at the top of your screen you’ll see Parts/Crew/Action Groups. Click on Action Groups and then click Custom01. Once you’ve done this, click on one of the solar panels and you’ll notice that they all glow a little blue. This means that you’ve added them to the Action Group but now you need to define what what the action group actully does. You’ll see on the right that you’ll get a bunch of options appropriate to the item you’ve selected. For the solar panels, select the “toggle” option. Once done, press save! Now, you’ll be able to deploy all your solar panels and retract them by pressing the 1 button at any time. Pretty handy when you’re taking off or making a re-entry attempt!

Hopefully you’ve found this guide of use! Let us know what you’d like to see covered next 🙂

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